In our cities the byproducts of combustion (in our engines and power plants) meet with the byproducts of life - plant resin off-gases, air-born dusts and sands, ocean sprays and sea salt mists - to create a complex mixture of chemicals that can be harmful to human health. As Gabriel Isaacman and I reported in the Atlantic last month, even so-called pristine landscapes are not free from this effect.

And that begs the question: what are we to make of natural sources of air pollutants? This was a question posed during the first day of the International Workshop for a Better EPI: Towards a Next Generation of Air Quality Monitoring, held last week at Seoul National University in Korea.

Small diameter particulate matter, so-called PM 10 (less than 10 microns in diameter) and PM 2.5 (less than 2.5 microns), are air pollutants of particular concern. These particles, which can be anything small, from grains of desert dust to particles of coal ash, are small enough to bypass the lung’s natural filters (hairs in your nose, mucus in your throat). They burrow deep into the vulnerable tissues of the lung, where whatever radioactive particles or heavy metals they brought with them can wreck havoc on easily damaged soft tissue.

All particulate matter, generally, is harmful to human health. At least according to Michael Krzyanowski of the World Health Organization, who presented on global PM monitoring efforts at the conference.

“Epidemiological studies have tried to separate the specific effects of the different components of PM,” he said, referring to efforts to clarify human health outcomes related to PM derived from cars from those related to PM derived from plants. “Either we aren’t there with research,” he said, “Or PM is just too complicated to separate.”

Because we can control the PM that is human-produced, either by limiting power plant emissions or driving fewer car miles, some participants at the conference argued that we should only consider these sources of PM in national inventories and performance metrics. Countries cannot, generally, limit the PM produced by an ocean breeze or a desert storm - they should not be penalized, or criticized, the argument goes, for having high PM levels from these sources.

These are fair criticisms. PM levels are currently a leading indicator in our Environmental Performance Index, which ranks the countries of the world on measures of environmental quality. And it is also a pollutant required for control under most rigorous air pollution programs around the world.

But there is evidence that natural PM sources are as harmful to human health as non-natural. In other words, “we don’t have evidence that non-anthropogenic PM is not-health relevant,” Krzyanowski said. As such, it is problematic to simply remove this source from environmental metrics.

“Health warnings should include Sahara dust in an index,” Krzyanowski argued, just as much as dust from construction or car emissions. “Yes that is hard to address in management. But it is still important for human health.”

There is no easy answer to this question – what pollutant sources to include in measurements and indices – but later versions of our own EPI may well seek to differentiate among these measures to create a more sophisticated measure of country-by-country managements obligations and public health risks.